Semi-random back-off method for achieving resource reservation in wireless local area networks

ABSTRACT

A method and apparatus are described including determining if a successful unicast transmission has occurred, adjusting a contention window to a minimum value and adjusting a time slot back-off counter to one-half of the value of the contention window plus one responsive to the determination and adjusting the contention window using one of a plurality of adjustment schemes and selecting said time slot back-off counter from an interval between zero and the contention window.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/US10/001962, filed 13 Jul. 2010, which waspublished in accordance with PCT Article 21(2) on 3 Feb. 2011 in Englishand which claims the benefit of European patent application No.EP09305710.7 filed 29 Jul. 2009.

FIELD OF THE INVENTION

The present invention relates to wireless communications in general and,in particular, to a semi-random back-off method for resource reservationfor providing increased throughput while reducing collisions overwireless local area networks (WLANs).

BACKGROUND OF THE INVENTION

In multicast/broadcast applications, data are transmitted from a serverto multiple receivers over wired and/or wireless networks. A multicastsystem as used herein is a system in which a server transmits the samedata to multiple receivers simultaneously, where the receivers form asubset of all the receivers up to and including all of the receivers. Abroadcast system is a system in which a server transmits the same datato all of the receivers simultaneously. That is, a multicast system bydefinition can include a broadcast system.

The popularity of voice and video applications over mobile computingdevices has raised concerns regarding the performance of medium accesscontrol (MAC) protocols, which are responsible for allocating sharedmedium resources to multiple communicating stations and resolvingcollisions that occur when two or more stations access the mediumsimultaneously. In the current IEEE 802.11 wireless LANs, thedistributed coordination function (DCF) of the MAC protocol layer uses abinary exponential back-off (BEB) algorithm for fundamental channelaccess. The BEB algorithm mitigates the issue of network collisions byrandomizing the timing of medium access among stations that share thecommunication medium. The timing of channel access in the BEB algorithmis randomized by setting the slot counter to a random integer selectedfrom contention window [0, CW] in each back-off cycle, and CW doublesupon failed data transmissions in last back-off cycle. Here a back-offcycle is a procedure where the back-off slot counter decrements downfrom an initial maximal value to zero. The simplicity and goodperformance of BEB contribute to the popularity of IEEE 802.11 DCF/EDCA.

However, as demonstrated by both practical experience and theoreticalanalysis, the BEB algorithm has some deficiencies. First, the collisionprobability for a transmission attempt increases exponentially with thenumber of active stations in the network. Second, the medium accessdelay cannot be bounded and the jitter is variable, which may not besuitable for multimedia applications.

Some concepts/terms that may benefit the understanding of the presentinvention are provided. A frame is a unit of data. That is, data can bepackaged in packets or frames or any other convenient format. As usedherein a frame is used to indicate data packaged in a format fortransmission. A back-off round/stage/cycle is a procedure in which theback-off slot counter counts down from an initial value (maximum) tozero. When the counter reaches zero, a new transmission is attempted.One frame transmission may involve multiple back-off rounds/stages(because of unsuccessful transmission attempts). As used herein a timeslot represents a continuous time period during which the back-off slotcounter is frozen. It may refer to either a fixed time period (usuallyseveral microseconds) sufficient for the physical layer to perform thecarrier sensing once, or a varying time period (usually between hundredsof microseconds to several milliseconds, depending on the length of thepacket and physical data rate) when a frame is being transmitted overthe shared medium. In a network with shared medium, each station freezesor decreases its back-off slot counter based on the resulting status ofthe physical or virtual carrier sensing of the medium. Hence, because ofthe shared nature of the medium, the slot count changes are alignedamong the stations. The time slot can be used as a basic time unit tomake the entire procedure discrete. Positive integers n=1, 2, 3, . . . ,N are used to indicate the 1^(st), 2^(nd), 3^(rd), . . . , N^(th) timeslot, and is used to indicate the status of the shared medium at the nthslot, for example, I_(n)=1 when busy and I_(n)=0 otherwise. The back-offslot count of station i at the nth time slot is denoted as slot_(i)(n).

In Application Serial Number PCT/US09/01179, a deterministic back-off(DEB) method was described to reduce or avoid collisions. In the DEBmethod, transmission were deterministically scheduled in time slots

In Application Serial Number PCT/US09/001,855, a relaxed deterministicback-off (R-DEB) method was described to overcome issues such asbackward compatibility and dependability that are inherent in thedeterministic back-off (DEB) method. The R-DEB method selects theback-off slot count in as deterministic a way as possible to reduce oravoid network collisions. The R-DEB method also introduces randomness tothis procedure to preserve the flexibility and easy deployment featureof the conventional random back-off methods such as the BEB (binaryexponential back-off) method. Hence, the R-DEB method made a compromisebetween the network efficiency and flexibility, and can be viewed as acombination of the DEB algorithm and BEB algorithm. The initialmotivation of the R-DEB algorithm was to adapt the deterministicback-off for video transport systems while maintaining backwardcompatibility with the previous standards.

The R-DEB operates as follows. A back-off round starts when a stationresets its back-off slot count slot(n) to the fixed number M (note thathere n is a variable on the timeline). Once it is determined by thephysical carrier sensing procedure that the sharing medium is idle for atime slot, the station decreases its back-off slot count by one. If thisnew slot count satisfies the transmission triggering condition (that is,the new slot count equals one of the elements of the triggering setQ_(T), e.g., slot(n)=k). The node (station, client device, mobileterminal, mobile device) will get an opportunity to initiate a datatransmission (hence “triggering a transmission”). If no frame is to besent at this time, the node forgoes the opportunity and continuesdecreasing its slot count. The result of the data transmissiondetermines whether or not the element k should further remain in thetriggering set: if there was a successful transmission then thistriggering element remain in the triggering set; if there was anunsuccessful data transmission then, with a probability p, a triggeringelement substitution procedure will be initiated that replaces the oldelement k with a new one k′ from the interval [0, M]. The R-DEB methodincluded a method and apparatus for selecting an element from theinterval [0, M−1] for inclusion in the triggering set Q_(T) to reducenetwork collisions. It should be noted that a station can be a computer,laptop, personal digital assistant (PDA), dual mode smart phone or anyother device that can be mobile.

The notion of resource reservation is a well known and widely used intime division multiple access (TDMA) schemes to achieve high throughputand a certain level of quality of service (QoS) provisioning forasynchronous transfer mode (ATM) networks. In a typical TDMA reservation(R-TDMA) solution, the radio resource is organized into superframes witheach superframe divided into multiple time slots of equal length, and astation can subscribe one or more time slots in each superframe as itsreserved radio resource by a reservation request. As used herein a node(client, mobile station, mobile device, mobile terminal, station,laptop, computer, personal digital assistant (PDA), dual mode smartphone, . . . ) are all terms that can be used to indicate an end devicein a wireless local area network (WLAN). A station can havecollision-free channel access in these reserved time slots, thus QoS canbe readily achieved. In another flexible version of R-TDMA, reservationALOHA (R-ALOHA), the radio resource is automatically reserved bytracking the results of data transmission in the previous superframe. Asuccessful transmission in a given time slot in the previous superframeresults in a reservation of the same slot for the sender (transmitter)in subsequent superframes. FIG. 1( a) illustrates how the reservation isachieved by R-ALOHA among three stations A, B and C. Station Asuccessfully obtains reserved slot l in the first superframe while B andC obtain their reserved slots at the second superframe because stationsB and C collide with each other in the first superframe. Simulationresults show that R-ALOHA achieves less delay at significantly higherchannel utilization than slotted ALOHA.

The beauty of R-AHOHA lies in that, it achieves resource reservationautomatically without the involvement of a central coordinator asconventional TDMA solutions usually do. The present invention borrowsthe idea of R-ALOHA and applies it to carrier sense multiple access(CSMA) based wireless networks. More specifically, the present inventionseeks to incorporate the R-ALOHA into distributed coordination function(DCF)/enhanced distributed channel access (EDCA) to improve the QoSperformance for content delivery over IEEE 802.11 networks in homeenvironments. The challenge is how to achieve resource reservation inthe context of CSMA environments.

Resource reservations in R-ALOHA are possible because of two basicfeatures in a TDMA network, periodicity and synchronization. Periodicitymeans that each superframe has the same number of equal-length timeslots so that a station can easily identify a time slot that it wants touse again in the current superframe. Synchronization makes time slotidentification possible because the time slot is exactly the same slotthe station used in the previous frame. In fact, as long as a networkpossesses the aforementioned two features, then resource reservation canbe achieved following a similar approach as R-ALOHA.

Now the problem that remains is to determine whether or not a CSMAnetwork possesses the two features—synchronization and periodicity.Generally, a CSMA network cannot be synchronized because of the presenceof hidden terminals in multi-hop networks. However, considering asingle-hop network where all stations share the same medium and assumingperfect carrier sensing, synchronization can be achieved.Synchronization in a CSMA network means that the back-off slot countersof stations in the network decrement or freeze simultaneously upon thesame network event (transmitting or idle) for a time slot. The term“time slot” in CSMA context has a different meaning from that used inTDMA context. It is defined as a continuous time period during which theback-off slot counter freezes. It may last a very short time period(usually several microseconds), sufficient for the physical layer toperform carrier sensing once, or a long time period (usually fromhundreds of microseconds to several milliseconds, depending on the framesize and physical data rate) sufficient to complete a frame exchangesequence. Thus the duration of a time slot in a CSMA network varies overtime.

Typically, for station A and B that share the same medium, the channelstate transition sequence learned by each station should be the samebecause of the broadcast property of wireless channels. More generally,if not considering hidden terminals, all stations sharing the mediumshould maintain the same sequence that reflects the utilization historyof the medium. This sharing property builds the notion ofsynchronization of back-off slot counters among stations in the samecollision domain.

To achieve periodicity in CSMA networks, deterministic selection ofvalues for the back-off slot counter is introduced in the back-offprocedure when a successful transmission occurred in last back-offcycle. Here a back-off cycle is a period of time when the back-off slotcounter decrements from the initial value to zero. By deterministicselection, a station's time slot counter is reset to a commondeterministic value shared by all stations in the network. This commonvalue defines the periodicity of CSMA network (in terms of time slots).

SUMMARY OF THE INVENTION

Herein, the issue of multiple access in IEEE 802.11 wireless networkswill be revisited.

FIG. 1( b) illustrates how resource reservation is achieved in CSMAnetworks. In the first back-off cycle, station A, B and C each generatesa random back-off slot counter as 1, 3, and 3 respectively. Sincestation A succeeds in data transmission in the 2nd time slot of thefirst back-off cycle, it resets its slot counter deterministically to 4after its data transmission, which allows it to access the 2nd time slotagain in next back-off cycle. Hence the second time slot is reserved forstation A. Station B and C collide at the 4th slot of the first back-offcycle, and stations B and C reset their slot counters to random values,for example 3 and 5 respectively, which allows them to access the 3rdand 5th time slots respectively in the next back-off cycle withoutcollision. Again, by setting their time slot counters to 4, both B and Ccan have reserved time slots for channel access in subsequent back-offcycles. The common value 4 gives the periodicity of network.

Therefore, in CSMA networks, resource reservation is achieved byintroducing periodicity into the back-off procedure. Periodicity relieson the low-level carrier sense mechanism to synchronize the network. Theperformance of resource reservation is affected by many factors, such ascarrier sense accuracy, hidden terminals, etc.

The present invention describes a method to achieve resource reservationin IEEE 802.11 wireless networks. In the method of the presentinvention, a reservation component is introduced into the back-offalgorithm which allows a station to deterministically set its back-offtime slot counter upon successful unicast transmission in a previouscycle. Resource reservation is achieved by reusing a time slot inconsecutive back-off cycles. This method can be applied in the currentIEEE 802.11 DCF/EDCA with least modification to the current softwareand/or hardware, but brings the extra advantage of reduced networkcollisions and thus higher throughput and less network delay. It isbelieved that multimedia applications including audio, video etc. willbenefit from the method of the present invention.

A method and apparatus are described including determining if asuccessful unicast transmission has occurred, adjusting a contentionwindow to a minimum value and adjusting a time slot back-off counter toone-half of the value of the contention window plus one responsive tothe determination and adjusting the contention window using one of aplurality of adjustment schemes and selecting said time slot back-offcounter from an interval between zero and the contention window.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Thedrawings include the following figures briefly described below:

FIG. 1( a) illustrates how the reservation is achieved by R-ALOHA amongthree stations A, B and C.

FIG. 1( b) illustrates how resource reservation is achieved in CSMAnetworks.

FIG. 2( a) shows the access procedure on discrete timeline.

FIG. 2( b) shows the access procedure on the service ring.

FIG. 3 is a flowchart of an exemplary semi-random back-off method forresource reservation in accordance with the principles of the presentinvention.

FIG. 4 is a block diagram of an exemplary embodiment of an apparatusimplementing the semi-random back-off method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. The Concept Of Semi-Random Back-Off (SRB)

Suppose that the frame for use in a network begins at time slot 0.Denote slot_(i)(n) as the value of back-off time slot counter of stationi at the n^(th) time slot. For a given transmission opportunity (TXOP),if all data transmissions are successful, indicated by acknowledgements,it can be said that this TXOP is positive; otherwise, it is said thatthis TXOP is negative. Note that in IEEE 802.11 MAC, broadcast and/ormulticast (B/M) data are usually not acknowledged so broadcast andmulticast data are treated as if a TXOP transmitting broadcast(multicast) data (B/M data) is negative.

Defining I_(txop) as the state of TXOP in last back-off cycle. Thesemi-random backoff (SRB) method of the present invention can bedescribed as follows. Whenever slot_(i)(n) reaches zero and needs to bereset, its value is updated by

$\begin{matrix}{{{slot}_{i}\left( {n + 1} \right)} = \left\{ \begin{matrix}M & {{{If}\mspace{14mu} I_{txop}} = {positive}} \\{{rand}\left( {0,{C\; W}} \right)} & {{{If}\mspace{14mu} I_{txop}} = {negative}}\end{matrix} \right.} & (1)\end{matrix}$where CW represents the back-off contention window as defined inconventional random back-off methods, and M is a common integer sharedamong stations in the network. Equation (1) states that, when theprevious TXOP is positive, which usually indicates a successfultransmission (for unicast only), then the slot counter is reset to adeterministic value M; otherwise, it is reset to a random value from theback-off contention window, as a conventional random back-off schemedoes.

One can realize that SRB contains a random component and a deterministiccomponent. The back-off procedure follows a conventional approach in therandom component but it introduces periodicity in the deterministiccomponent. A station chooses which component to reset its back-off timeslot counter after the station determines whether a TXOP is positive ornegative in previous back-off cycle.

In Equation (1), CW is a time-varying parameter among stations that canbe controlled by different random back-off methods. For example, thechange of CW can follow a binary exponential increase when applying SRBto DCF/EDCA, or follow other varying patterns like exponential increaseexponential decrease (EIED) or linear increase linear decrease (LILD).On the other hand, M is a fixed parameter for a station used forreservation. In the context of DCF/EDCA, M is set to (CW_(min)+1)/2 soas to keep the average delay of SRB equal to the smallest delay inbinary exponential back-off (BEB). M can also be different for stations,in which case the service ring size is defined by the greatest commentdivisor (gcd) of different M in the network.

The parameter M in the deterministic component of SRB defines theperiodicity to reserve channel resources. The access procedure in SRBcan be best described using a service ring including M fixed time slots.Determining a value for the back-off time slot counter is equivalent toselecting a time slot from the ring. When using the random component forback-off, the time slot is randomly selected from the ring, whereas whenusing the deterministic component for back-off, the time slot isreserved because the back-off interval equals the ring size which allowsthe method of the present invention to repeatedly use the same time slotin subsequent back-off cycles.

FIG. 2( a) shows the access procedure as a discrete timeline while FIG.2( b) shows the access procedure as a service ring. When station Acollides at time slot A1, it resets its back-off counter to a randomvalue, which allows station A access the channel at time slot A2. Sincestation A was successful in transmitting data at time slot A2, station Adeterministically sets its back-off counter to M. On the service ring,setting the back-off time slot counter to M means a revisit of theprevious slot. That is, time slots A2 and A3 correspond to the same sloton the ring. Thus resource reservation is achieved. For stations A andB, as long as they reserve different time slots on the ring, stations Aand B can have collision-free channel access in the network. Herein, thedeterministic component is also called the reservation component.

The role of the service ring in the SRB is similar to that of thesuperframe in the R-ALOHA. The primary difference between them is thatin SRB the length of a time slot is variable while in R-ALOHA it isfixed. SRB should be more efficient than R-ALOHA because the time wastedin idle is minimized since an idle time slot has the shortest length intime.

Although the principle of SRB is described in terms of single-hopnetworks without any error, it can also be applied to multi-hop anderror-prone networks, at the cost of reduced performance gain. Asdiscussed, resource reservation is best achieved in single-hop networkswithout carrier sense error. However, for a multi-hop and error-pronenetwork, the presence of hidden terminals, carrier sense errors and thechannel errors can impair resource reservations—the former two factorslead to an unsynchronized back-off procedure among stations thatdestroys the synchronization. The last factor results in poor resourcereservation performance because most TXOPs are recognized as negative.In such circumstances, the SRB uses the random component for back-off.Thus, the performance gain due to resource reservation would be reduced.

When new stations join the network, a new station first uses the randomcomponent to select a time slot for data transmission, which may causenetwork collisions for the first several back-off cycles after the newstation joins the network. However, the collision probability decreaseswith network evolvement because more and more stations would usereserved time slots for data transmission.

2. Semi-Random Back-Off in a General Case

It has been assumed, thus far, that each station in the networkmaintains the same parameter M for resource reservation. Since stationshave a service ring with the identical size, as long as a stationchooses a time slot different from other stations in the service ring asits reserved time slot, then each station will experience collision-freechannel access. However, when stations have service rings of differentsizes, or stations maintain a different reservation parameter M, a timeslot reserved by a station may still collide with other reserved slotsof other stations. Consider two stations i and j in a single-hop networkwith different reservation parameter M_(i) and M_(j) respectively.Suppose that in a certain back-off cycle, both have collision-freechannel access and, thus, stations i and j enter into the reservationstate in subsequent back-off cycles. For a time slot in the currentback-off cycle, say time slot l, the time slot counters of the twostations should be different from each other (to allow collision-freechannel access in this cycle), slot_(i)(l)≠slot_(j)(l). It is necessaryto find what value of M_(i)/M_(j) would lead to collision-free reservedtime slots for station i and j in subsequent back-off cycles. Note thatthe service ring size is M_(i) for station i and M_(j) for station j.

After k time slots, the value of time slot counters of station i and jwould becomeslot_(i)(k+l)≡slot_(i)(l)−k mod M_(i)  (2)slot_(j)(k+l)≡slot_(j)(j)−k mod M_(j)  (3)To allow collision-free channel access, station i and j should holddifferent time slot counters in all time slots subsequent to slot l,i.e. slot_(i)(k+l)≠slot_(j)(k+1) for all k≧0. This impliesa·M+slot_(i)(l)≠b·M+slot_(j)(l) for ∀a,bεZ  (4)From congruence theory, this is equivalent to saying that d_(ij) cannotbe divided by (slot_(i)(l)−slot_(j)(l)), where d_(ij) is the greatestcommon divisor (gcd) of M_(i) and M_(j). Therefore, it is believed that,

-   Lemma I. To have collision free reserved slots between station i and    j, slot_(i)(l) and slot_(j)(l) should fall in different congruence    classes modulo d_(ij)(=gcd(M_(i),M_(J))).    For a N-station network with reservation parameters M₁, M₂, . . . ,    M_(N), Lemma I should follow for any pair of stations. Denote    slot_(i)(l) (1≦i≦N) as the values of time slot counters at a time    slot l, then by the congruence theory,-   Theorem I. Let d=gcd (M₁, M₂, . . . , M_(N)), then the sufficient    condition to have collision-free reserved slots in the system is    that slot_(i)(l) (1≦i≦N) should fall in different congruence classes    modulo d.

The factor d defines the size of a common service ring in the network,and the service ring of each station is a multiple of such commonservice ring. Note that slot_(i)(l) (1≦i≦N) is determined by the randomselection component of SRB. Thus, as long as two stations selectdifferent time slots from the common service ring as their reservedslots, then they would have collision-free channel access in subsequentback-off cycles. Moreover, in some cases, two stations can multiplex atime slot in the common service ring without collision.

The discussion on different sizes of service ring in the network is ofparticular use when applying SRB in the IEEE 802.11 EDCA, where data ofdifferent priorities using different reservation parameter M for channelaccess.

3. SRB in IEEE 802.11 DCF/EDCA

Next, how to apply the concept of SRB by integrating SRB into theprevalent MAC IEEE 802.11 DCF/EDCA protocols is discussed. Since bothprotocols build on the BEB (binary exponential back-off), the focus ison how to adapt the concept of SRB to the BEB mechanism. For clarity,the prefix S is used to refer to a method with SRB capability, likeS-BEB, S-DCF and S-EDCA.

As discussed above, the major difference between SRB and a generalrandom back-off method is that a reservation component is introduced inSRB in addition to the usual random component. By adding a reservationcomponent, BEB evolves to S-BEB. In S-BEB, the reservation parameter Mis set to (CW_(min)+1)/2. Note that in BEB the average interval betweenimmediate successful transmissions is (CW_(min)+1)2 time slots, thuschoosing (CW_(min)+1)/2 for M keeps the average delay of S-BEB inreservation state comparable to the smallest delay (on average) in BEB.

Pseudo-code for S-BEB is as follows.

 1 If last transmission was successful Then  2   CW ← CW_(min)  3   Ifit was a unicast transmission Then  4     $\left. {{slot}_{i}(n)}\leftarrow{\left\lfloor \frac{{{CW}\mspace{14mu}\min} + 1}{2} \right\rfloor\mspace{70mu}\text{/}*{reservation}\mspace{14mu}{component}*\text{/}} \right.$ 5   Else  6     slot_(i)(n) ← rand(0, CW)  7   End If  8 Else  9   CW =min(CW × 2 + 1, CW_(max)) 10   slot_(i)(n) ← rand(0, CW) 11 End if

In the pseudo-code, lines 3 and 4 deal with the reservation component.

FIG. 3 depicts a flowchart of an exemplary method for resourcereservation in accordance with the principles of the present invention.Further, the exemplary processes illustrated in at least FIG. 3 and textbelow are operationally implemented in either the host processing systemor the wireless communication module or a combination of the hostprocessing system and the communication module. The block diagram (FIG.4 described below) thus fully enables the various methods/processes tobe practiced in hardware, software, firmware, a field programmable gatearray (FPGA), an application specific integrated circuit (ASIC), areduced instruction set computer (RISC) or any combination thereof. At305 the user (station, node, client device, mobile terminal, mobiledevice) has its back-off counter initialized to a random value from theinterval [0, CW_(min)]. At 310 the back-off counter is decreased by oneupon each idle time slot, and whenever it reaches zero, a datatransmission commences. At 315 a test is performed to determine if thelast transmission was a successful unicast transmission. If the lasttransmission was not a successful unicast transmission, then thecontention window (CW) size is adjusted at 320 following a conventionalway as used for IEEE 802.11 DCF/EDCA or other methods. For example, CWcan be tuned according to the results of last transmission byexponential increase exponential decrease (EIED), linear increase lineardecrease (LILD), multiplicative increase linear decrease (MILD), etc.The back-off counter is randomly drawn from the interval [0, CW].Otherwise if the last transmission was a successful unicasttransmission, then the contention window CW is reset to CW_(min), andthe back-off counter is set to a deterministic value (CW_(min)+1)/2 at315. A further test is performed at 330 to determine whether thenode(station, user, client device, mobile device, mobile terminal) hasmore data to send. If it has more data, then the processing continues at310. Otherwise, this processing ends. What makes the method of thepresent invention semi-random is the concept that the setting of theslot count to a random number in the interval (0, CW) occurs only ifthere is an unsuccessful transmission indicated by no acknowledgementfrom the receiver. If there has been a successful transmission at acertain time slot in previous back-off cycle, then the user (station,node, client device, mobile device, mobile terminal)) reserves that timeslot at step 325 at the current back-off cycle. It should be noted thatwhile a scheme involving incrementation is used herein, such a scheme isexemplary and a decrementation scheme could be used just as easily.

As in EDCA CW_(min) varies for different access categories (ACs), thereis also a different reservation parameter M for these ACs. There arefour access categories in EDCA as defined in the standard: voice (VO),video (VI), best effort data (BE) and background data (BK). When usingthe default settings in IEEE 802.11e EDCA, CW_(min)(VO)=7,CW_(min)(VI)=15, CW_(min)(BE)=31 and CW_(min)(BK)=31. Hence thereservation parameter for each access category should beM_(VO)=(7+1)/2=4, M_(VI)=(15+1)/2=8, M_(VO)=(31+1)/2=16 andM_(VO)=(31+1)/2=16. Clearly the common divisor should be d=4, which isthe reservation parameter for voice applications.

It is worth noting that although in the description the parameter M isfixed for a station or an access category, it can also be a time-varyingvalue that changes adaptively to the network congestion level. In thiscase, many methods that are used to evaluate the network congestionlevel can be employed here. For example, if the number of activestations in the network is estimated to be S, then M can bea×(CW_(min)+1)/2, where “a” is the minimal integer that satisfiesa×(CW_(min)+1)/2≧S. Obviously, “a” changes over time. Such a scheme canbe readily integrated into SRB by a small change to the reservationcomponent in SRB. As in FIG. 3, it is only necessary to change step 325to say that the back-off counter is reset to a×(CW_(min)+1)/2 (insteadof (CW_(min)+1)/2).

A salient feature of the proposed method is that stations in a singlecollision domain can be served in a round robin manner because eachsubscribes to a dedicated time slot from the service ring.

The block diagram of FIG. 4 may be implemented as hardware, software,firmware, a field programmable gate array (FPGA), an applicationspecific integrated circuit (ASIC), a reduced instruction set computer(RISC) or any combination thereof. Further, the exemplary processesillustrated in the various flowcharts and text above are operationallyimplemented in either the host processing system or the wirelesscommunication module or a combination of the host processing system andthe communication module. The block diagrams thus fully enable thevarious methods/processes to be practiced in hardware, software,firmware, a field programmable gate array (FPGA), an applicationspecific integrated circuit (ASIC), a reduced instruction set computer(RISC) or any combination thereof.

FIG. 4 is a block diagram of an exemplary embodiment of an apparatusimplementing the semi-random back-off method of the present invention.The present invention can be implemented in either an end user device ora server, base station (BS), access point (AP) or other controller. Tothat end the block diagram is the same or similar for all such devicesand an exemplary embodiment is shown in FIG. 4. Since the presentinvention can be implemented in software the host computing system isshown with a CPU and memory which can be used in combination to storeand execute instructions to perform the method of the present invention.The CPU could just as easily be replaced by or augmented by hardware orfirmware including an ASIC, a FPGA, a RISC processor or any otherappropriate combination of hardware, software or firmware. The hostcomputing system communicates with a wireless communication module viaan input/output (I/O) interface. The device communicates with otherdevices via the wireless communication module which includes a mediaaccess control and baseband processor, radio transmitter/receiver and atleast one antenna.

It is to be understood that the present invention may be implemented invarious forms of hardware, software, firmware, special purposeprocessors, or a combination thereof. Preferably, the present inventionis implemented as a combination of hardware and software. Moreover, thesoftware is preferably implemented as an application program tangiblyembodied on a program storage device. The application program may beuploaded to, and executed by, a machine comprising any suitablearchitecture. Preferably, the machine is implemented on a computerplatform having hardware such as one or more central processing units(CPU), a random access memory (RAM), and input/output (I/O)interface(s). The computer platform also includes an operating systemand microinstruction code. The various processes and functions describedherein may either be part of the microinstruction code or part of theapplication program (or a combination thereof), which is executed viathe operating system. In addition, various other peripheral devices maybe connected to the computer platform such as an additional data storagedevice and a printing device.

It is to be further understood that, because some of the constituentsystem components and method steps depicted in the accompanying figuresare preferably implemented in software, the actual connections betweenthe system components (or the process steps) may differ depending uponthe manner in which the present invention is programmed. Given theteachings herein, one of ordinary skill in the related art will be ableto contemplate these and similar implementations or configurations ofthe present invention.

The invention claimed is:
 1. A method, said method comprising:determining if a successful unicast transmission has occurred; if asuccessful unicast transmission has occurred, adjusting a contentionwindow to a minimum value and adjusting a time slot back-off counter toone-half of the value of said contention window plus one; adjusting aservice ring size responsive to said time slot back-off counter; andadjusting said contention window using one of a plurality of adjustmentschemes and selecting said time slot back-off counter from an intervalbetween zero and said contention window.
 2. The method according toclaim 1 wherein said adjustment schemes include binary exponentialback-off, multiplicative increase linear decrease, exponential increaseexponential decrease and linear increase linear decrease.
 3. The methodaccording to claim 1 further comprising: determining if there is moredata to transmit; and transmitting said data responsive to saiddetermination.
 4. An apparatus comprising: means for determining if asuccessful unicast transmission has occurred; means for adjusting acontention window to a minimum value and adjusting a time slot back-offcounter to one-half of the value of said contention window plus one if asuccessful unicast transmission has occurred; means for adjusting aservice ring size responsive to said time slot back-off counter; andmeans for adjusting said contention window using one of a plurality ofadjustment schemes and selecting said time slot back-off counter from aninterval between zero and said contention window.
 5. The apparatusaccording to claim 4, wherein said apparatus is one of a mobile device,a server and a base station.
 6. The apparatus according to claim 4,further comprising: means for determining if there is more data totransmit; and means for transmitting said data responsive to saiddetermination.